1. Field of the Invention
The present invention relates to an angular velocity sensor to be used, for example, in a car navigation system, a control system for an automobile, or a manual blur correcting system for still or video cameras, and to a process or method for manufacturing the angular velocity sensor.
2. Prior Art
A principal angular velocity sensor known in the prior art is the expensive and large top-type gyroscope which is most often used in airplanes or ships. In recent years, however, there has been developed an inexpensive oscillatory type angular velocity sensor, the application of which has been expanded to automotive control systems, car navigation systems as well as to manual blur correcting systems for video or still cameras. As a result of these expanded and new uses, the demand for smaller and less expensive angular velocity sensors has greatly increased.
Accordingly, there has been proposed in recent years an angular velocity sensor employing a single crystalline piezoelectric element made of quartz or lithium tantalate, making it possible to provide a smaller and less expensive angular velocity sensor.
In the prior art angular velocity sensor utilizing a single crystalline piezoelectric element, a pair of arms are joined and fixed at their individual end portions by a root member to form a tuning fork oscillator. This tuning fork oscillator is, for example, cut from a quartz sheet into an integral structure. A set of drive electrodes are affixed to one of the arms of the tuning fork oscillator for driving the tuning fork oscillator piezoelectrically in the direction of a principal plane at a resonant frequency. The drive electrodes are electrically driven by an external oscillator circuit. A monitor and a ground electrode are affixed to the other arm for detecting the oscillation amplitude caused by the oscillator circuit. Sense electrodes are also affixed to the same arm as the monitor electrode for piezoelectrically detecting the stress due to the Coriolis force acting on the tuning fork.
In the prior art sensor, the electric charge generated at the monitor electrode is amplified by an external circuit and then compared with a reference signal preset by an AGC (auto gain control), to control the oscillator circuit which maintains the oscillation amplitude of the tuning fork oscillator constant. On the other hand, the sense electrodes detect a signal due to the Coriolis force, which is amplified by an external amplifier circuit synchronously with the signal detected by the monitor electrode. The modulated detected signal is thereafter demodulated and the undesired frequency band is filtered out by an LPF (low pass filter).
Angular velocity sensors of this type have not been effective for a number of reasons. For example, quartz is typically a single crystalline piezoelectric material composed of arrayed single crystals of SiO2. Since SI has a positive polarity and O2 has a negative polarity, electric neutralization is established by arraying SI and O2 symmetrically. However, if a strain is applied, to the SiO2 piezoelectric material the electric symmetry is broken and electric charges are generated.
FIG. 30A is a diagram representing the individual axes of a quartz crystal. As shown, an axis joining the edges is defined as an X-axis (or electric axis) and an axis normal to the plane made by the Xxe2x80x94X axes is defined as a Z-axis (or optical axis). FIG. 30B represents a section in the X-Y plane having the polarities shown. As described above, a single crystalline piezoelectric material, such as quartz, exhibits specific piezoelectric characteristics and has specific polarities with respect to the crystal axes which depend on the molecular array of the crystalline piezoelectric material.
On the other hand, an oscillation type angular velocity sensor detects rotation of an object by detecting the Coriolis force acting at a right angle with respect to the direction of the oscillations. Thus, the angular velocity sensor is required to have piezoelectric characteristics for two axes and to have means for applying the oscillations and means for detecting the force or oscillations at a right angle to the applied oscillations. Generally, a tuning fork oscillator is cut from a piezoelectric material in the direction as shown in FIG. 31. Although it is easy to apply the driving oscillations in a tuning fork oscillator which has been cut in the direction shown in FIG. 31, the sensitivity of the detected Coriolis force (the perpendicular oscillations) is very low.
Thus an object of the invention is to provide an angular velocity sensor which has an enhanced detection sensitivity via the sense electrodes.
In accordance with another object of the invention, there is provided an angular velocity sensor which has a simple electrode construction which can be formed by, for example, a photolithographic method.
In order to solve the above-specified problems, according to the invention, there is provided an angular velocity sensor which comprises a bimorph structure including a first tuning fork member including at least two arms formed from a single crystalline piezoelectric material and at least one root member joining the arms, and a second tuning fork member having a shape generally similar to that of the first tuning fork member and also formed from a single crystalline piezoelectric material. The first tuning fork member and the second tuning fork member are directly bonded in the thickness direction into an integral bimorph tuning fork oscillator structure with the crystal axes directions of the piezoelectric materials of the two tuning fork members having inverse polarities with respect to each other in the width direction.
More specifically, the first tuning fork member and the second tuning fork member have reversed polarities in the electrically active X-direction. Therefore, it is necessary to provide only a single set of sense electrodes formed on opposed peripheral faces of one of the arms of the tuning fork oscillator. If an electric field is applied in the X-axis direction, the field direction, and the polarized piezoelectric material directions are aligned on one turning fork member surface of one arm but are reversed on the second turning fork member surface, the extending/contracting forces in the opposite directions act in opposition to the oscillations, which act in parallel with the plane normal to the X-axis direction. In contrast, in the prior art devices, a charge component due to the Coriolis force is canceled in the vicinity of the center of the section of the arm. By inverting the polarity of the piezoelectric material, as described, the direction of the charges caused by the Coriolis force, in the section of the arm,-greatly enhance the efficiency of the angular velocity sensor. Thus, a factor which weakens or decreases the electric field intensity in the prior art angular velocity sensors is reduced thereby enhancing the detection sensitivity of the angular velocity sensor of the invention. By this arrangement, the bimorph structure of the present invention has a Coriolis force detection sensitivity which is twice as high as the prior art sensors. Thus, because the axial direction in which the Coriolis force is to be detected is also the axial direction in which the piezoelectric characteristics are high, the resulting angular velocity sensor has a high sensitivity and an excellent total S/N ratio.
Still further, in the angular velocity sensor of this invention, the drive electrodes for causing oscillation in parallel with the principal plane of the tuning fork oscillator are preferably formed on the four outer peripheral faces of one of the two arms of the tuning fork and the sense electrodes for detecting the electric charge generated by oscillations normal to the principal face of the tuning fork oscillator are preferably formed on the opposed side faces of the other arm of the tuning fork, thereby, simplifying the electrode construction compared with the prior art sensors.
According to still another aspect of the invention, there is provided an angular velocity sensor comprising a tuning fork oscillator including first and second arms made of a single crystalline piezoelectric material, and at least one root member joining the first and second arms. The first and second arms are bonded such that the polarities of the crystal axes of the first and second arms are perpendicular to each other in the thickness direction of the tuning fork oscillator. Since the polarities of the piezoelectric material are perpendicular in the two arms of the bimorph tuning fork of the present invention, the charge components due to the Coriolis force, as detected from the two arms, are in phase with each other but in opposite phase with respect to the acceleration component. This makes it easy to connect the sense electrodes with the common electrode so that the amplification stages of the amplifier can be simplified. Another advantage of this construction is that the phase shift of the two independent amplifiers of the prior art and the problem of the drift of the intermediate voltage due to the temperature characteristics can be avoided.
As a still further advantage of the invention, with the polarities of the piezoelectric materials differing between the two arms (or at the center of), the tuning fork, one arm is preferably used for driving the tuning fork and for monitoring the drive operation and the other arm is used for the sensing the Coriolis force and monitoring the sensing operation or for only sensing the Coriolis force. As a result, it is possible to efficiently process the two orthogonal oscillations piezoelectrically, i.e., the drive of the tuning fork oscillator and the detection of the stress due to the Coriolis force.
According to a further aspect of the invention, there is provided an angular velocity sensor comprising two tuning fork members made of piezoelectric sheets of crystals which exhibit a piezoelectric phenomena and inverted piezoelectric phenomena and have a plus polarity at one end of the electrically active X-axis thereof and a minus polarity at the other end, and including a pair of arms extended from the tuning fork root members to set the X-axis direction in the width or thickness direction, in which the two tuning fork members are directly bonded with the polarities of the X-axis direction being inverted from each other.
In a further aspect of the invention, the angular velocity sensors as described, may have sense electrodes which are not divided but are formed as a single structure so that the extending/contracting forces in the opposite directions are applied to the individual arms of the tuning fork oscillator which are inverted in polarity so that the detection sensitivity can be enhanced.
According to a still further aspect of the invention, there is provided a process for manufacturing an angular velocity sensor wherein the tuning fork members are formed by photolithography from a sheet material which is prepared by bonding at least two sheets of single crystalline piezoelectric material directly in different crystal axes directions. As a result, the oscillator can be manufactured extremely inexpensively and can be mass produced.
According to yet a further aspect of the invention, there is provided a process for manufacturing an angular velocity sensor wherein the tuning fork members are formed by photolithography from at least two wafers of single crystal piezoelectric materials which are directly bonded on the wafers in different azimuths of the crystal axes. By bonding the tuning fork members which are etched prior to bonding, even a thick tuning fork oscillator can be manufactured by the photolithographic method so that the aforementioned technique can be utilized even for applications where high precision and high S/N ratio are required.